A nuclear chain reaction is obtained by bombarding fissionable fuel material with neutrons from a source of neutrons to split some of the atomic nuclei of the fuel into fragments, thereby releasing useful energy in the form of heat and other neutrons for bombarding other nuclei, and so on, ad infinitum, so long as each fuel nucleus undergoing fragmentation produces a minimum of one neutron on the average which in turn fragments another fuel nucleus. The process, called fission, depends for its continuity on an adequate supply of neutrons and fuel. To control the reaction, devices having large neutron absorption cross sections are generally utilized in conjunction with a moderating material for slowing down neutrons so that they possess the desired energy spectrum. The fuel, neutron absorbing material and moderator, together with associated structural components make up the core of the reactor, through which a heat transfer fluid is circulated to remove heat generated by the fission process.
The present invention is concerned with a structural component of the core, in that it is concerned with apparatus for spacing and supporting nuclear fuel bearing members in an upright position within the core of a nuclear reactor.
To achieve optimum power distribution across the core, consideration must be given to the shape and distribution of the fuel bearing members. Since the heat generated by the fuel is more efficiently dissipated from the area surrounding the fuel when the fuel bearing member has a high ratio of surface area to volume, modern day reactors utilize a plurality of slender elongated fuel members or rods known as fuel pins which are each made of a length of fuel material enclosed in a relatively thin-walled circular tube of cladding material. A number of fuel pins are clustered together in a composite assembly known as a fuel assembly or bundle, whereas a plurality of fuel bundles make up a core. Due to their slender construction, whatever means is used to space and position the fuel pins relative to one another must have good mechanical stability as well as the ability to compensate for the slight differences in cross-sectional dimension of the individual fuel pins due to manufacturing tolerances. In addition, to expose the greatest possible surface area of a given pin to coolant flow, contact between the supporting structure and the pin, and obstructions to coolant flow around the pins, should be minimized.
Since fuel pins are quite long compared to their diameters, one or more spacer grids are provided to prevent bowing of the fuel pins due to thermal, mechanical and hydraulic influences, and to maintain them in the desired array.
Spacer grids are subject to conflicting design objectives. The grid must have sufficient strength to limit fuel rod bowing and vibration and to resist severe thermal and hydraulic forces. It must provide sufficient contact area with the fuel rods to minimize local fretting damage to the fuel rod cladding at the points of contact. It must accommodate fuel rod swelling and it must allow fuel rod insertion without damage. The grid should require, on the other hand, a minimum of material to minimize parasitic neutron absorption. It should be designed to minimize restriction of coolant flow through the channel and it should be adapted to fabrication from low-neutron absorption materials. In addition, it should be structurally sound without dependence upon critical fabrication processes.
The grids of the present invention may be used with nuclear reactor fuel bundles such as are used in pressurized water reactors (PWRs), boiling water reactors (BWRs) or light water breeder reactors (LWBRs), although it is especially useful for LWBRs. In such bundles, fissionable fuel is disposed within the elongated fuel pins which are mounted in a parallel array, generally between a pair of end plates. When the bundle is in service in a nuclear reactor, water or other coolant passes along the outer surface of the fuel pins, receiving heat generated therein. In order to permit uniform heat removal and to avoid overheating of the rods, it is necessary that they be accurately spaced apart laterally. Maintaining the spacing of the elongated rods in such a closely spaced array under conditions in which vibration naturally occurs, requires a carefully designed spacer. Fuel pin spacers are typically made of some type of spring steel or zircaloy strips interlocked at rights angles to form an egg crate. The egg crates are typically spot welded at the strip intersection to provide structural connections. Such spacers have been the subject of many prior art patents as exemplified below.
U.S. Pat. No. 3,423,287 to Anthony et al describes a spacer for providing lateral support and spacing for fuel pins in elongate fuel bundles such as are commonly used in PWR cores.
U.S. Pat. No. 3,463,703 to Crandall describes a spacer grid consisting of parallel strips of jointed "V's" transverse of and between the fuel pins, each V containing a fuel pin which is supported by protuberances extending from the side of the V.
U.S. Pat. No. 3,679,547 to Warberg describes a nuclear reactor fuel assembly having spacer support grids formed from grid strips which are interconnected at right angles to form a grid cell for supporting each fuel pin by means of four resilient supporting points acting in opposing pairs.
U.S. Pat. No. 3,920,515 to Ferrari et al describes a fuel assembly spacer grid comprised of interwoven metallic straps formed into an egg-crate grid having opening for receiving fuel pins. Spring and dimple support structures are provided at the fuel pin-spacer grid interface.
U.S. Pat. No. 3,933,583 to Jabsen also describes an egg-crate grid structure formed of inter-connected metal strips for supporting fuel pins in a nuclear reactor core.
The above-referenced patents all relate to grid assemblies for either BWR or PWR cores. The physics requirements for a LWBR demands that the fuel pins be more closely spaced together than the fuel pins of a PWR or BWR. Therefore, the generally square grid lattices discussed above are not as efficient or practical as a more tightly spaced lattice arrangement. Note, however, the U.S. Pat. No. 3,679,547 discussed above was proposed for use with a breeder reactor utilizing liquid sodium or water as a coolant.
A hexagonal shaped grid structure (like a honeycomb) has been proposed for use with LWBRs. Hexagonal shapes, however, cannot conveniently be formed into interlocking strips such as in grid structures described above.
In U.S. Pat. No. 4,021,300 to Marshall et al a fuel assembly spacer grid is proposed wherein the grids are attached to a hexagonal channel (as opposed to grid lattice). The fuel pins are supported by a grid spacer formed from a staggered pattern of layers of two "line-of-sight" wave beams retained within the hexagonal channel. The grid thus formed is not of the interlocking variety and could not be applied in a closely-spaced hexagonal pin lattice which does not have an adequate "line-of-sight" between the rows of fuel pins. While the benefits in terms of ease of fabrication and construction of spacer grids formed from interlocking strips is well known, to date, no configuration has been devised for forming a spacer grid, having a hexagonal fuel pin array, from interlocking strips.